During the resistance welding of metal sheets, electric currents with a high current strength are introduced into the sheets by two electrodes pressed against the outer surfaces of the sheets to be welded to each other. By this means, the metal of the sheets is melted and forms a weld nugget which connects adjacent sheets fixedly to each other. The welding electrodes are customarily composed of copper or copper alloys.
In particular in the case of aluminum resistance welding, the ideal shape and the cleanliness of the surface of the welding electrodes are a substantial prerequisite for producing a spot weld of high and reproducible quality. The surfaces of the welding electrodes can be impaired by deposits and wear even by welding a few spot welds, for example ten to twenty spot welds, and therefore the spot welds produced do not have the desired strength.
For this reason, welding electrodes are reworked at regular intervals such that the surfaces thereof have an optimum shape and are free from contaminations during each welding operation. A known method for reworking the surfaces of the welding electrodes is the grinding of the welding electrodes.
In order to grind the ends of flexible bristles of brushes, the document DE 297 13 087 U1 discloses a device in which a conical grinding wheel rotates about an axis of rotation and at the same time is pivotable in a manner revolving about a second axis running obliquely with respect to the axis of rotation. The angle of slant of the second axis corresponds to the angle of taper of the grinding wheel, and the flexible bristles are pressed against the grinding wheel substantially at the intersecting point of the second axis with the surface of the grinding wheel. Because of the angle of slant and angle of taper being identical in value, this results substantially in a horizontally running surface of the grinding wheel in the area of contact with the bristles when the second axis runs vertically. By means of the revolving pivoting of the grinding wheel, the direction of the grinding movement of the surface of the grinding wheel can be changed, and therefore the ends of the flexible bristles, which are bent in the direction of rotation of the grinding wheel by the latter, are ground from all sides, and a grinding result with bristle ends which are approximately hemispherically rounded is produced. Such a procedure is not possible in the case of solid, non-flexible workpieces, since the latter are not bent by the grinding force.
The company AMDP S.A.S. from Croissy-sur-Seine provides a grinding device for welding electrodes, in which a flat grinding wheel is fastened to a rigid rotary spindle which is set into rotation by a drive motor. The drive motor is located in a housing, the upper end of which is fastened pivotably to a frame. A wobble-type drive moves the lower end of the housing on a circular path, and therefore the rotary spindle accommodated in the housing executes a wobbling movement in order to produce a conical electrode surface on the electrode with the grinding wheel. This device has a positionally fixed position of the wobbling grinding wheel, and therefore the welding electrodes have to be pressed against the grinding wheel, for example by means of a robot arm carrying the welding tongs with the electrodes, for the machining. This requires highly complicated programming of the robot arm.
Accordingly, it would be desirable to provide an improved device and a method of the abovementioned type for solid workpieces, which automatically permit the production of surfaces which are not flat. If the machined workpiece is a welding electrode, the device and the method are intended to permit, at as high a speed as possible, the production of a conical or cap-shaped electrode surface, which is optimally suitable for the welding operation.